1) Field of the Invention
The invention relates to sheet-like radiation-sensitive recording materials. More specifically, the present invention is directed to sheet-like radiation sensitive recording materials providing with an image in a contact printing process, and on whose surface a discontinuous dulling layer is present, and a process for applying the dulling layer.
2) Description of Related Art
Dulling layers are conventionally used, in particular, in contact printing processes. In these processes, the original is placed directly on the radiation-sensitive recording material, usually in a so-called vacuum contact printing frame. The direct contact between original and recording material is produced by sucking out the air present in between. In the case of very smooth surfaces, it generally takes a long time before a constant vacuum is reached. In addition, air occlusions may remain behind, leading to sidelighting. Many recording materials also contain compounds which undergo a chemical reaction on exposure to radiation and evolve gases, for example nitrogen. The removal of such gases also presents problems in the case of smooth surfaces.
These disadvantages can be substantially avoided if copying materials having a rough surface are used. Such a surface can be achieved, for example, by dusting the radiation-sensitive layer with a finely divided powder. However, as a rule this leads to a nonuniform roughness. Moreover, the powder may become detached from the surface and contaminate the vacuum pump.
A rough surface is also obtained if the recording material is embossed with a roller.
Inorganic or organic particles of a certain size which may be added to the solution or dispersion which serves for the production of the radiation-sensitive layer project from the finished layer after drying and likewise result in a rough surface. If these particles are present in the printing layer, the term "micro-pigmentation" is used; however, if they are on the copying layer, the term "dulling" is used.
Dulling layers can be divided into those which completely cover the surface (=continuous dulling layers) and those which only partly cover the surface (=discontinuous dulling layers). The discontinuous layers frequently comprise more or less hemispherical protuberances (="culottes").
DE-A 30 03 131 (=U.S. Pat. No. 4,288,521) discloses dusting with the aid of a cyclone and subsequent thermal or chemical fixing of solid dulling agents for producing discontinuous layers.
According to DE-A 26 06 793 (=U.S. Pat. No. 4,216,289), dulling is possible by applying a coating solution via an embossing roll to a coating roll which comprises resilient material having a fine, uneven pattern. From this coating roll, the coating solution is then transferred to a support.
DE-A 31 31 570 (=GB-A 2 081 919) discloses that an aqueous liquid which contains a resin in dissolved or dispersed form can be sprayed on. Spraying can be supported by applying an electrostatic voltage of at least 5 kV (EP-A 0 344 985, EP-A 0 649 063).
In all these processes, however, the height, diameter and number of dulling particles per unit area are subject to certain fluctuations, so that the behavior during contact imaging--i.e., placing a positive or negative master in a vacuum contact frame on the dulled radiation-sensitive layer, evacuating the frame and irradiating through the master--is not reliably reproducible.